1. Field of the Invention
The invention relates to a light irradiation apparatus and an inkjet printer. The invention relates especially to a light irradiation apparatus which forms a line-like, narrow light irradiation region on the article to be irradiated with light, and irradiates it with light, and to an inkjet printer in which such a light irradiation apparatus is installed and with which images are recorded on a recording medium.
2. Description of the Prior Art
Currently, for example, an inkjet recording method is used for various printing tasks such as reprinting, for example, photography printing, marking color filters and the like, because pictures can be made by an engraved printing method easily, and moreover, at low cost.
In an inkjet printer using such an inkjet recording method, pictures with high image quality can be produced by a write head which regulates and delivers fine dots which are suitably combined with ink by color reproduction process, in which permanency, output suitability and the like have been improved, and with special paper with ink absorption properties, color material-color generation properties, surface shine and the like which have been suddenly improved.
Generally, inkjet printers can be classified based on the types of ink used. For example, an inkjet printer of the photosetting type is known in which ink of the photosetting type is used which is set by light irradiation, such as UV radiation or the like. (See, for example, Japanese Patent Application Publication JP-A 2005-246955 (corresponding to U.S. Patent Application Publication 2005/168509 A1); Japanese Patent Application Publication JP-A 2005-103852; Japanese Patent Application Publication JP-A 2005-305742; and the publication “Trends in UV inkjet printing”, Hiromichi Noguchi, Teruo Orikasa, Journal of the Japanese Society of Printing Science, 2003, Volume 40, no. 3, pp. 32 to 46.)
The inkjet method of the photosetting type has the advantages of a relatively minor smell and usability, besides for special paper, also for a recording medium which dries quickly and does not have absorption properties.
One type of inkjet printer using such an inkjet method of the photosetting type is shown in FIG. 13 and has:                a write head 71 which is provided with a nozzle (not shown) which, for example, applies ink of the UV radiation setting type to the recording medium R as extremely small droplets, and        a head part 70 in which two light irradiation apparatus 80A, 80B are installed in a carriage 72, and which are located, for example, on opposite sides of the writing head 71 for setting the ink which has been applied to the recording medium R with UV radiation.        
The head part 70 is supported by a rod-like guide rail 75 which extends along the recording medium R and is moved back and forth by a drive device which is not shown in the drawings along the guide rail 75 above the recording medium R.
The inks of the UV radiation setting type to be used can be, for example, the following and similar ones:                ink based on radical polymerization which contains a radically polymerizable compound as a polymerizable compound; and        ink based on cationic polymerization which contains a cationically polymerizable compound as a polymerizable compound.        
In this connection, the ink based on radical polymerization plays the main role.
The light irradiation apparatus 80A, 80B, each have a box-shaped cover component 81 with a light exit opening 81A which is open in the direction in which the recording medium R is located (in FIG. 13 toward the bottom). In this cover component 81, a discharge lamp 82 of the long arc type which forms a linear light source is arranged such that it extends parallel to the recording medium R in the direction which orthogonally intersects the direction of motion of the head part 70. With respect to the light exit opening 81A, behind the discharge lamp 82 there is a bucket-like reflector 83 with a reflection surface 83A in the form of an ellipsoid which extends along the discharge lamp 82 and reflects the light from the discharge lamp 82, the discharge lamp 82 being arranged at the first focal point Fr1 of the reflection surface 83A. The discharge lamp 82 is, for example, a high pressure mercury lamp or a metal halide lamp, the length of the light emitting part having a size with which a light irradiation region IA is formed which, for example, is larger than the dimensions (width dimension) of the recording medium R in the direction of motion of the head part 70.
In the above described inkjet printer, the head part 70 is arranged such that the recording medium R is positioned at a second focal point Fr2 of the reflector 83 for the light irradiation apparatus 80A, 80B or in the vicinity of the second focal point Fr2. By moving the head part 70 above the recording medium R, for example, in the state in which the discharge lamp 82 remains in operation, the light from the discharge lamp 82 is emitted and linearly focused on the recording medium R which is positioned at the second focal point Fr2 of the reflector 83. In this way, the ink of the UV radiation setting type is set directly after it has been applied to the recording medium R.
The following is a specific description of the setting treatment of the ink of the UV radiation setting type (irradiation treatment of the ink of the UV radiation setting type with UV radiation).
In FIG. 13, the ink of the UV radiation setting type which has been applied to the recording medium R is set by the radiant light of one of the light irradiation apparatus, specifically the light irradiation apparatus 80A, which is positioned downstream of the direction of motion of the head part 70 when printing takes place in the recording medium R by the head part 70 being moved to the right, for example. On the other hand, the ink of the UV radiation setting type applied to the recording medium R is set by the radiant light of the other light irradiation apparatus 80B which is positioned downstream of the direction of motion of the head part 70 when printing takes place in the recording medium R by the head part 70 being moved to the left in FIG. 13.
Recently there have been the corresponding demands for raising the picture quality obtained from an inkjet printer using the above described inkjet recording method of the photosetting type and a demand for faster setting treatment of the ink. The reason for this is the following.
As is shown in Japanese Patent Application Publication JP-A 2005-246955 corresponding to U.S. Patent Application Publication 2005/168509 A1 described above, for example, ink based on radical polymerization has the property that the presence of oxygen reduces the concentration of radicals. The longer the ink is exposed to the atmosphere, the lower the rate of setting and the greater the time consumption for ink setting. The ink used in an inkjet printer must have a somewhat low viscosity so that it is easily discharged from the nozzle of the write head. When the ink has not immediately set (photopolymerized) after application of the ink to the recording medium, due to the frequent changes of the dot form of the applied ink, a picture with high image quality can no longer be obtained.
Based on this requirement, it can be considered that, for example, by increasing the peak irradiance of the light emitted by the light irradiation apparatus, the polymerization reaction will be accelerated.
In the above described publication “Trends in UV inkjet printing”, it is shown that, for example, by using a microwave lamp with high irradiance, the amount of reduction of the rate of ink setting by oxygen can be reduced, i.e., that a reduction of image quality can be prevented by rapid execution of the ink setting treatment. For example, an equally large light irradiation region can be formed such as by a discharge lamp of the long arc type. Furthermore, the utility of a microwave UV lamp is shown in which still higher irradiance is obtained than in a discharge lamp of the long arc type. The peak irradiance of the microwave UV lamp which is described in publication “Trends in UV inkjet printing” is, for example, roughly 1000 mW/cm2 to 1200 mW/cm2.
In the Japanese Patent Application Publication JP-A 2005-103852, a technique is described in which there is a cylindrical lens between a light source lamp and a recording medium which focuses light from the light source lamp and irradiates the recording medium with it, and in which, thus, the peak irradiance of the light emitted onto the recording medium is increased.
Even if using optical elements, such as lenses, mirrors and the like, the light from the light source lamp is focused and thus irradiated, there is a limit to the magnitude of the peak irradiance that can be obtained since the radiance of the light source lamp, itself, has not been increased.
The same applies to the case of using a microwave UV lamp. In order to obtain irradiance which is relatively high relative to the degree to which the above described requirement can be met, it is necessary to increase the radiance of the microwave UV lamp in itself. However, in reality, it is technically difficult to further increase the radiance of a lamp of the long arc type and a microwave UV lamp that has light emitting parts which are large.
Moreover, in the above described inkjet printer, there are the following disadvantages:
In a conventional inkjet printer, for example, with the arrangement shown in FIG. 13, the light exit openings 81A of the light irradiation apparatus 80A, 80B, and the light exit opening 83B of the reflector 83 open in the same direction. This means that the recording medium R is directly irradiated with light from the discharge lamp 82 which contains light from the visible range to the IR range which is unnecessary for setting of the ink of the UV radiation setting type, and moreover, also radiant heat is incident on the recording medium R in the course of operation of the discharge lamp 82. The recording medium R is heated by the light and the radiant heat from the visible wavelength range to the IR wavelength range so that a high temperature is reached.
Since, in many cases, the recording medium R is one that can be easily deformed by heat, such as, for example, paper, resin, film and the like, the amount of influence of heat on the recording medium R by unnecessary light from the visible range to the IR range and by the radiant heat of the discharge lamp in the case of using a discharge lamp becomes large because a high peak irradiance is to be obtained in order to carry out setting treatment of the ink of the UV radiation setting type with high efficiency. The recording medium R is thus shifted into a still higher temperature state, by which there are the disadvantages that deformations and the like often arise and it becomes difficult to form pictures with high image quality.
In view of these disadvantages, it can be imagined that there is a reflection mirror (also called a cold mirror) between the discharge lamp and the recording medium which reflects only light with wavelengths which are necessary for the setting of the ink and on which a film is formed by vapor deposition which transmits light with other wavelengths, so that the recording medium is irradiated only with light which is reflected by this reflection mirror and that thus the heat effect on the recording medium is reduced.
However, in the case of an arrangement of such a reflection mirror, the length of the optical path between the discharge lamp and the recording medium is increased by which, for example, focusing is impossible in the case of a discharge lamp of the long arc type with respect to the lengthwise direction of the discharge lamp. The area irradiated with light is increased in this way so that the degree of light utilization is reduced, and at the same time, it becomes impossible to obtain a relatively high irradiance on the light irradiation area.
As was described above, in reality, for an inkjet printer using the inkjet method of the photosetting type, an improvement of the setting treatment of the ink could not be achieved by using an inkjet printer with high radiance of the light source lamp in itself, and thus, an increase of the peak irradiance on the irradiated surface could not be achieved.